The addition of clandestine opioids (e.g., fentanyl, acetylfentanyl, and carfentanil) to heroin produces an extremely potent drug mixture believed to fuel the striking increase in overdose deaths in the US. Commonly available drug tests, however, do not detect these clandestine opioids. Nimble and reproducible surveillance methods are urgently required to keep pace with the emergence of clandestine opioids, inform clinical practice, and guide immediate public health interventions. An ideal surveillance strategy would utilize advanced laboratory methods (capable of identifying known and previously unidentified clandestine opioids), while simultaneously incorporating survey methods to provide critical contextual information for the accurate interpretation of the laboratory test results, The goal of this R21 project is to develop and refine a sentinel process for the rapid identification of clandestine opioids (called Sentanyl) in individuals who present to the emergency department (ED) after an opioid overdose. We will evaluate the application of cutting edge analytical laboratory techniques that have a robust capacity to detect a broad range of both known and previously unidentified clandestine opioids. We also will develop a survey instrument to obtain critical context for the correct interpretation of drug testing results. The project will culminate in pilot testing of a scalable and reproducible process for surveillance of clandestine opioids, with rapid dissemination of information to experts in public health, public safety, and public policy. The project will provide essential preliminary data for a subsequent R01-level, multi-site comprehensive evaluation of the impact of the clandestine opioid surveillance system. The specific aims for this innovative, critically needed, early-stage, developmental R21 are to: 1. Implement and evaluate advanced testing methods that identify clandestine opioids after an opioid overdose. We will implement an enhanced analytical toxicology approach to urine drug testing to identify currently known and not yet identified clandestine opioids among 250 adult ED patients who present after naloxone reversal for signs of an opioid overdose. This innovative approach will allow us to detect the presence of potentially dozens of known, and also yet to emerge, clandestine opioids. 2. Develop a process for contextualizing advanced urine drug testing results through drug user-derived data. We will conduct semi-structured interviews among adult ED patients presenting after opioid overdose who provide concurrent urine samples for testing;? we will then use the results of these interviews to develop an audio computer-assisted self- interviewing (ACASI) survey instrument, and refine it through an iterative process among subsequent study participants. 3. Pilot test our process for the detection of emerging clandestine opioids and surveillance of known clandestine opioids. Building on Aim 1 and Aim 2, we will integrate the drug testing results and user-derived data for each participant. We will also create a database for tracking cases, and make our de-identified data available for communication with clinicians, public health and safety authorities, and policy makers. In addition, we will refine our processes, and devise a manual of procedures to allow reproducibility of our Sentanyl process to allow for more widespread application.